Atrial fibrillation (AF) and ventricular tachycardia (VT) affect millions of patients in the United States. These arrhythmias can be cured with catheter ablation, but the arrhythmias often recur, and these recurrences are generally due to reversible conduction block from incomplete ablation. The inability to confirm the presence of completely ablated lesions in the desired locations is the major factor in the greater than 40% recurrence of VT after ablation, and the greater than 30 % recurrence of AF after ablation. In addition, it is no possible with current technology to adequately predict the pathways of VT through scar, which are the targets for ablation. The overall goal of the program is to use advanced, image-based technologies to improve targeting and assessment of electrophysiology intervention. In the initial 5-year funding period, the goals were to develop needed technology, define the clinical system, and demonstrate the feasibility of the approach. Those goals were achieved. In this renewal funding period, the goals are to use the developed technology in clinical studies of VT ablation, continue to improve the technology, and expand the scope into clinical studies of AF ablation. We hypothesize that high-resolution Magnetic Resonance Imaging (MRI) with compatible electrode catheters, location sensors, mapping systems, real-time scanner control, and computational modeling, can (1) aid in predicting the locations of arrhythmia circuits (2) aid in predicting the locations of critical ablation targets, (3) provide for accurate catheter navigationto those critical targets, (4) monitor the formation of ablation lesions in real time, and (5) assess the completeness of ablation. Once validated, these enhanced capabilities could dramatically improve the outcomes from complex ablation procedures, become the ablation methodology of the future, and become a platform for improving outcomes from many other interventions. In the current program, we developed important, innovative methods, and MRI-compatible versions of ablation equipment, for predicting VT ablation targets, for performing ablations in an MRI scanner, and for lesion imaging. We developed imaging methods that differentiate incompletely ablated (reversibly damaged) tissue from completely ablated (necrotic) tissue. This allows determination of whether there is complete lesion necrosis, or whether additional ablation is needed during the procedure to complete the ablation, and, thereby, reduce recurrences. In this new proposal, we will apply these innovative technologies to clinical ablation studies, since they already represent a substantial improvement over current methods. We will continue development of improved technologies for full realization of the potential for MRI guided ablation. We will expand our previous focus on ablation of VT to ablation of AF, and emphasize studies of procedure efficacy. This project is a partnership between the Johns Hopkins University Departments of Medicine, Radiology, and Biomedical Engineering; Sheba Medical Center; and industrial partners: St Jude (impedance-based tracking system), Greatbatch (catheter components), and Siemens (intra-procedural computational modeling).